Fire refining of nickel-containing metallurgical intermediates and scrap



United States Patent Int. c1. c2211 1/12, 23/00 US. Cl. 75-23 20 Claims ABSTRACT OF THE DISCLOSURE A process is disclosed for recovering nickel or nickelcopper alloys from sulfide materials which can contain copper in amounts such that the nickel to copper ratio is at least about 3 :7, which process includes surface blowing a turbulent bath of the sulfide material with a gas containing free oxygen to lower the sulfur content to less than about 4% and to introduce sufiicient oxygen into the turbulent bath to react with the remaining sulfur and then subjecting the turbulent bath to a subatmospheric pressure of less than about 0.1 atmosphere to remove substantially all of the sulfur as sulfur dioxide.

The present invention relates to an improved process for the smelting and refining of nickeland nickel-coppercontaining sulfide materials for the direct recovery of metallic nickel and nickel-copper alloys therefrom.

Thermodynamically and kinetically the reactions between nickel sulfide, oxygen and nickel oxide in a converter allow full conversion of nickel matte to metallic nickel. Although entirely feasible theoretically, a commercially operable process based on these reactions had not been developed until the recent Queneau and Renzoni proposal described in US. Pat. No. 3,069,254. By employing a mechanically-induced turbulent bath and oxygenenriched air or oxygen, they showed that the direct conversion of nickel sulfide was commercially possible. After removal of iron by oxidation and slagging, top blowing and mechanical agitation of the molten bath is continued until the sulfur content is lowered to about 2% and the temperature of the agitated bath is raised to about 3000 P. so that the remaining sulfur and oxygen incorporated therein can readily react. Final desulfurization in the process described in the aforesaid patent is achieved by partial combustion of fuel to maintain temperature and to continuously remove sulfur dioxide from above the turbulent bath to maintain a low partial pressure of sulfur dioxide and thereby drive the reaction of sulfur and oxygen in the bath toward completion. This purging treatment works well but is a relatively slow process.

It has now been discovered that the Queneau and Renzoni process can be further improved by continuous smelting and by achieving final sulfur elimination more rapidly. Nickeland nickel-copper-containing materials such as mattes, matte concentrates and scrap can be directly and continuously converted to liquid nickel or nickelcopper alloys by surface blowing a turbulent bath-of the molten material and by vacuum treatment for final desulfurization. Furthermore, it has been found that the content of impurities such as lead, zinc, cadmium, bismuth and antimony can be lowered to extremely low levels. It has also been found that the process of the present invention can provide a product with a low content of dissolved gases, particularly suited for continuous casting.

It is an object of the present invention to provide a 3',5 l 6,8 Patented June 23, 1970 "ice pyrometallurgical process for the direct and continuous production of nickel and nickel-copper alloys from sulfidic materials containing nickel and copper.

An even further object of the present invention is to provide a process for increasing the kinetics of the final desulfurization of a molten bath of nickel and nickelcopper alloys.

Other objects and advantages will become apparent from the following description.

Generally speaking, the present invention contemplates surface blowing a molten bath of a nickel-containing sulfidic material with a stream of a free-oxygen-containing gas while maintaining the bath in a turbulent state to lower the sulfur content to less than 4% and to incorporate oxygen into the turbulent bath in an amount more than stoichiometrically suflicient to form sulfur dioxide from substantially all the sulfur in the molten bath. The molten bath containing the remaining sulfur and the incorporated oxygen is then subjected to a vacuum treatment at a pressure of less than about 0.1 atmosphere while maintaining the bath in a state of turbulence to remove impurities and substantially all of the remaining sulfur.

Nickel-containing and nickel-copper-containing sulfide concentrates, mattes or matte concentrates having a nickel to copper ratio of at least about 3:7 can be treated in accordance with the process of the present invention but the invention is not limited thereto. For instance, sulfurcontaining residues from refining processes and other nickel-containing metallurgical intermediates can also be treated in accordance with the process of the present invention. Scrap containing nickel and copper can also be treated. Of course, any combination of the foregoing materials can be employed. The aforementioned materials can be treated in accordance with the process of the present invention starting from either the solid or liquid state. It is to be understood that the term nickel-containing sulfidic material as used herein refers to all of the aforedescribed materials but is not limited thereto.

Because in accordance herewith, final desulfurization is conducted in a separate vacuum unit, the demands placed on the apparatus in which the preponderant part of the sulfur is removed are not as stringent as required by prior art processes. Thus, during this phase of the process, the molten sulfide bath can be maintained in a turbulent state by pneumatic means as Well as by mechanical agitation or electromagnetic stirring.

In carrying the invention into practice, a turbulent bath of molten nickel-containing sulfidic material is established. After the removal of any iron present by oxidation and slagging, a portion of impurities such as lead, Zinc, cadmium, bismuth and antimony and a preponderant part of the remaining sulfur are removed by directing a stream of a free oxygen-containing gas such as air, preheated air, oxygen-enriched air or commercial oxygen upon and into the surface of the bath. If the sulfide material being treated is solid, it is advantageously added substantially continuously to the molten bath while surface blowing with commercial oxygen. The advantages of this technique are: (1) increased efiiciency of the overall operation by eliminating a separate melting operation, (2) increased sulfur dioxide concentration in the offgas due to the use of commercial oxygen which can be employed when the exothermic heat of converting reaction is absorbed by smelting solid feed, and (3) substantially constant sulfur dioxide content of the off-gas for more efiicient utilization thereof. Surface blowing of the turbulent bath is discontinued when desulfurization has proceeded to the point where oxygen in the amount required to oxidize the remaining sulfur is incorporated in the bath. Advantageously, this amount of oxygen is dissolved in the bath. However, if the required amount of oxygen cannot be dissolved in the bath, additional oxygen can be incorporated as finely dispersed nickel oxide. The molten bath is monitored to determine the sulfur and oxygen contents thereof so that surface blowing can be discontinued before significant interference by formation of nickel oxide dross occurs.

As the sulfur content is lowered to 4%, advantageously lower than about 2%, e.g., about 0.5%, the process is controlled so that the temperature of the turbulent bath attains a minimum value of between about 1200 C. and 1600 C., which minimum temperature is directly proportional to increasing nickel to copper ratios ranging from about 3:7 upwards. As the sulfur content is lowered to 4%, advantageously less than about 2%, oxygen as dissolved oxygen and, if necessary, as finely dispersed nickel oxide is incorporated in the bath to oxidize substantially all of the remaining sulfur. For more complete sulfur elimination, the amount of oxygen incorporated in the molten bath is more than stoichiometrically sufficient to oxidize substantially all the sulfur in the bath to sulfur dioxide and, advantageously, the amount of oxygen incorporated in the bath corresponds to the formula [%O]=[%S]+X wherein [%O] is the amount of oxygen incorporated in the bath, [%S] is the amount of sulfur in the bath and X is a number from about 0.1 to about 1.

At this point the molten bath is subjected to a vacuum treatment for impurity removal and final sulfur elimination. It is desirable for thermodynamic and kinetic reasons to subject the molten bath to a vacuum at a pressure of less than about 0.1 atmosphere and advantageously to a pressure less than about 1 millimeter of mercury. If the bath is deficient in oxygen, additional oxygen-bearing materials can be added thereto. Advantageously, gaseous oxygen can be introduced into the molten bath to overcome any oxygen deficiency. The addition of gaseous oxygen during the low pressure treatment is advantageous in that it lessens the need to incorporate all the oxygen required for final desulfurization during the surface blowing treatment and therefore avoids the problems associated with possible formation of undesirable nickel oxide dross.

In order to fully realize the improved thermodynamics and kinetics of the vacuum treatment for final sulfur elimination and impurity removal, the molten bath must be maintained in a turbulent condition. For kinetic reasons and to insure substantially complete sulfur elimination, e.g., down to about 0.05% sulfur and advantageously to below about 0.01% sulfur, the temperature of the agitated molten bath is maintained at a minimum between about 1200 C. and about 1600 C. with the minimum temperature being directly proportional to increasing nickel to copper ratios in the bath ranging from about 3:7 upwards. In addition to rapid sulfur elimination, a further advantage of the vacuum treatment is that other truoblesome impurities, such as bismuth and lead can be eliminated almost to nondetectable amounts.

After final desulfurization, the molten nickel-containing bath can be deoxidized by the addition of carbon, silicon, aluminum or calcium silicon. Deoxidation can also be effected by treating the turbulent bath with a reducing atmosphere containing carbon monoxide or hydrogen or by passing a reducing gas containing carbon monoxide, hydrogen or methane through the molten bath. Advantageously, deoxidation is effected under reduced pressures so that after deoxidation is completed the residual content of dissolved gases such as hydrogen can be lowered.

It has been observed that metal produced in accordance with the process disclosed in US. Pat. No. 3,069,254, often boils when being cast. This has occurred even at low sulfur contents. The cause of this boil is not presently known but it may be due to evolution of sulfur dioxide or dissolved oxygen upon cooling. This evolution of gases presents difficulties even for conventional casting practice and this metal would be wholly unsuitable for continuous casting.

The low pressure treatment of the present process and the turbulence of the molten bath provide highly efificient degassing so that the degassed metal can be cast and even continuously cast without encountering the problems described above.

Final desulfurization, deoxidation and degassing can be conducted in a suitable vacuum chamber in which low pressures are maintained by mechanical pumps, steam ejector system, or any other system capable of pumping large volumes of gas at low pressures. The vacuum chamber is equipped with means for controlling the temperature of the molten bath. For instance, the vacuum unit can be heated by induction or by carbon arc or other means. The molten bath can be maintained in a state of turbulence while undergoing vacuum treatment by electromagnetic stirring or by pneumatic or mechanical means.

A particularly advantageous embodiment of the present invention is to surface blow a molten turbulent bath of low sulfur content while substantially continuously adding solid feed to the bath. In practice, a turbulent molten nickel-containing bath having less than about 4% sulfur is established and maintained at a minimum tem perature between about 1200 C. and 1600 C. which minimum temperature is directly proportional to increasing nickel to copper ratios in the bath ranging from about 3:7 upward. The turbulent bath is surface blown with commercial oxygen while nickel-containing sulfidic material including scrap is substantially continuously fed to the bath, at a rate such that the sulfur content and temperature of the bath remain substantially constant. The advantage of this technique are: (1) increased efficiency of the overall operation by eliminating a separate melting operation; (2) improved kinetics within the converting unit, the low sulfur bath providing a heat sink, which assists the rapid melting and solution of the solid feed, and an oxygen sink, which possesses a high activity of oxygen and thereby the rate of sulfur oxidation; (3) less back-contamination of the finished melt due to low sulfur content of material absorbed by the refractory lining; (4) increased sulfur dioxide concentration in the off-gas due to the use of commercial oxygen which can be employed when the exothermic heat of converting is absorbed by smelting solid feed; and (5) substantially constant sulfur dioxide content of the off-gas for more efiicient utilization thereof. Prior to tapping, feeding is discontinued but surface blowing with commercial oxygen or partially combusted fuel is continued to further lower the sulfur content and to incorporate into the bath an amount of oxygen sufficient to oxidize the sulfur remaining therein. Surface blowing is then discontinued and a part of the molten bath is transferred to a vacuum chamber as described hereinbefore. The addition of nickel-containing sulfidic material to the part of the bath remaining in the furnace and the surface blowing with commercial oxygen thereof are then resumed.

For the purpose of giving those skilled in the art a better understanding of the invention, the following illustrative example is given:

EXAMPLE I A one-ton molten bath is established in a rotary furnace having an inside diameter of about 2 meters and is maintained in a turbulent condition by rotating the furnace at 20 revolutions per minute. The bath contains 4% sulfur, smaller amounts of copper, cobalt and iron, the balance being essentially nickel. The temperature of the bath is 1600 C.

Commercial oxygen of 99% purity is directed upon and into the surface of the bath at a rate of 200 cubic feet per minute (1 atmosphere and 60 F.). A solid nickel matte concentrate containing 71.2% nickel, 0.9% copper, 0.7% cobalt, 0.2% iron, 0.04% lead, 0.001%

bismuth, 26.0% sulfur and small amounts of silica is continuously added to the bath at a rate of about 1.15 tons per hour for 6 hours. During this period, the sulfur dioxide content of the ofi-gas averages about 58%. At the end of this period, the bath weighs about 6.14 tons and contains about 1.2% sulfur. The temperature of the bath is 1640 C.

Feeding of the matte concentrate is discontinued while surface blowing is continued with a mixture of commercial oxygen and natural gas in a volume ratio from 1:1 to 1.5 :1. This procedure increases the temperature to 1650" C. and continues desulfurization while preventing formation of nickel oxide dross. In 45 minutes, the sulfur content is lowered to about 0.5 and about 0.9% oxygen is dissolved in the bath. Analyses reveal the lead content is down to 0.002% and bismuth is down to 0.0004%.

The molten nickel is transferred to a vacuum unit where its temperature is maintained at 1650 C'by inductive heating. Electromagnetic stirring is employed to maintain the bath in a turbulent condition. The pressure in the chamber is lowered to 0.1 mm. of mercury over a period of 30 minutes and is maintained at this level for another minutes.

The nickel is then cast into sound pigs with no problems due to gas evolution. The final sulfur content of the metal is 0.02% while the lead content is reduced to 0.000l% and bismuth is not detected.

It is to be noted that all percentages, parts and ratios given herein are on a weight basis unless otherwise specified.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. A process for desulfurizating a sulfidic material selected from the group consisting of nickel-containing and nickel-copper-containing sulfidic materials which comprises surface blowing a molten bath of the sulfidic material with a stream of a free oxygen-containing gas while maintaining the bath in a turbulent state to lower the sulfur content to less than 4% and to incorporate in the bath an amount of oxygen more than thoichiometrically sufiicient to form sulfur dioxide from substantially all of the sulfur remaining therein and then subjecting the bath to a vacuum treatment at a pressure of less than about 0.1 atmosphere to remove impurities and substantially all of sulfur dioxide formed by the reaction of oxygen and substantially all the sulfur remaining in the bath while maintaining the bath in a turbulent state.

2. A process as described in claim 1 wherein the free oxygen-containing gas is selected from the group consisting of air, preheated air, oxygen-enriched air and commercial oxygen.

3. A process as described in claim 1 wherein as the sulfur content of the bath is lowered to about 2% the temperature of the bath is controlled to have a minimum temperature between about 1200 C. and about 1600 C., said minimum temperature being directly proportional to increasing nickel to copper ratios ranging from about 3:7 upward.

4. A process as described in claim 3 wherein the sulfur content is lowered to about 0.5% by surface blowing.

5. A process as described in claim 1 wherein the vacuum treatment is at a pressure of less than about 1 millimeter of mercury.

6. A process as described in claim 1 wherein the vacuum treatment is conducted at a minimum temperature between about 1200 C. and about 1600 C., said minimum temperature being directly proportional to increas- 6 ing nickel to copper ratios ranging from about 3:7 upward.

7. A process as described in claim 1 wherein a freeoxygen containing gas is introduced into the turbulent molten bath during the vacuum treatment.

8. A process as described in claim 1 wherein the molten bath of sulfidic material is surface blown with commercial oxygen and solid nickel-containing sulfidic material is added substantially continuously to the molten bath.

9. A process as described in claim 1 wherein the amount of oxygen incorporated into the turbulent bath corresponds to the formula:

wherein [0%] is the amount of oxygen incorporated in the bath, [8%] is the amount of sulfur remaining in the bath and X is a number ranging from about 0.1 to l.

10. A process as described in claim 1 wherein after substantially all the sulfur has been removed the turbulent molten bath is deoxidized by passing therethrough a reducing gas while maintaining the turbulent bath under a vacuum at a pressure of less than about 0.1 atmosphere.

11. A process as described in claim 10 wherein the vacuum treatment is continued after deoxidation with said reducing gas to degas the tubulent molten bath.

12. A process for desulfurizing a sulfidic material selected from the group consisting of nickel-containing and nickel-copper-containing sulfidic materials which comprises establishing in a furnace a turbulent molten nickelcontaining bath having less than about 4% sulfur; maintaining the turbulent bath at a minimum temperature between about 1200 C. and 1600 C., said minimum temperature being directly proportional to increasing nickel to copper ratios which range from about 3:7 upward; surface blowing the turbulent bath with commercial oxygen while substantially continuously adding a nickelcontaining sulfidic material to the turbulent bath; prior to tapping, discontinuing the addition of nickel-containing sulfidic material while continuing surface blowing to incorporate into the turbulent bath an amount of oxygen more than stoichiometrically sufficient to oxidize the sulfur remaining therein; tapping a part of the turbulent bath with said amount of oxygen incorporated therein; subjecting the tapped part of the bath to a vacuum treatment at a pressure of less than about 0.1 atmosphere to remove substantially all the sulfur while maintaining the bath in a turbulent state; and continuing surface blowing of the part of the turbulent path remaining in the furnace while resuming the addition of nickel-containing sulfidic material.

13. A process as described in claim 12 wherein the sulfur content of the turbulent bath is lowered to less than about 2% by surface blowing after the addition of nickel-containing sulfidic material is discontinued.

14. A process as described in claim 12 wherein the sulfur content of the turbulent bath is lowered to less than about 0.5% by surface blowing after the addition of nickel-containing sulfide material is discontinued.

15. A process as described in claim 12 wherein the vacuum treatment is at a pressure of less than about 1 millimeter of mercury.

16. A process as described in claim 12 wherein the vacuum treatment is conducted at a minimum temperature between about 1200 C. and about 1600 C., said minimum temperature being directly proportional to increasing nickel to copper ratios ranging from about 3:7 upward.

17. A process as described in claim 12 wherein a free oxygen-containing gas is introduced into the turbulent molten bath during the vacuum treatment.

18. A process as described in claim 12 wherein the amount of oxygen incorporated into the turbulent bath corresponds to the formula:

wherein [0%] is the amount of oxygen incorporated in the bath, [S%] is the amount of sulfur remaining in the bath and X is a number ranging from about 0.1 to 1.

19. A process as described in claim 12 wherein after substantially all the sulfur has been removed the turbulent molten bath is deoxidized by passing therethrough a reducing gas While maintaining the turbulent bath under a vacuum at a pressure of less than about 0.1 atmosphere.

20. A process as described in claim 19 wherein the vacuum treatment is continued after deoxidation with said reducing gas to degas the turbulent molten bath.

References Cited UNITED STATES PATENTS FOREIGN PATENTS 10/1948 Canada.

9/1956 Great Britain.

HENRY W. TA'R RING II, Primary Examiner US. Cl. XJR. 

